Exclusive: Sources say California Senate leader to challenge Sen. Dianne Feinstein in Democratic primary

De León has begun calling labor leaders and elected officials to inform them of his plans, the sources said, and is expected to soon announce his campaign against Feinstein, a giant of California Democratic politics who has held the office since 1992.

The 50-year-old de León, who represents Los Angeles and is seen as a leading Latino voice in Democratic politics, is likely to campaign aggressively against President Donald Trump. He began signaling he could oppose Feinstein in late August, after she said Trump could "be a good president" and that he "can learn and change." Feinstein later clarified that she is "under no illusion that it's likely to happen and will continue to oppose his policies."

He will enter the race after a state legislative session in which California lawmakers pushed legislation targeted at Trump -- making California a "sanctuary state," sought to force Trump to release his tax returns by making it a requirement to appear on the state's ballots as a presidential candidate, and passed a resolution urging Trump "to publicly apologize to all Americans for his racist and bigoted behavior."

He likely won't be the only Feinstein challenger. Tom Steyer, the billionaire environmentalist and single largest Democratic donor, is "very much looking at the Senate race," a source close to him said. Steyer, who is in Washington this week, issued a letter Wednesday challenging Democratic candidates to call for Trump's impeachment.

"This is no time for 'patience' --- Donald Trump is not fit for office. It is clear for all to see that there is zero reason to believe 'he can be a good president,'" Steyer said in the first line of his letter.